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As the title says, how to tell if an exe will load a DLL statically or dynamically by looking at the PE file header?
In other words, how to tell if the DLL is part of the executable, or will be called by the loader?
Thanks
Let me first clarify some terminology to avoid confusion.
Anything executed within a DLL is by definition dynamic. But, a DLL may be statically bound or dynamically bound to an executable.
With static binding, the EXE links against a DLL's import library (actually a .LIB file that is built alongside the DLL). Corresponding DLL function prototypes in header files will usually be declared with __declspec(dllimport). This results in the EXE being filled with stubs for each DLL symbol that are filled in by the Windows loader at runtime. This is facilitated by the final EXE having an import section structure in its PE headers listing all the DLLs to be resolved by the Windows loader at runtime and their symbolic names (e.g. functions). Windows then does all the dirty work to find and load these DLLs and referenced symbolic addresses before the EXE starts execution of the primary thread at its entry point. If Windows fails to find any DLL(s) or referenced symbolic addresses, the EXE won't start.
With dynamic binding, the EXE explicitly invokes code to load DLL(s) and resolve symbolic addresses. This is done using the two KERNEL32 API functions: LoadLibrary() and GetProcAddress(). If an EXE does this, there will be no associated import section describing this DLL and its symbols, and the Windows loader will happily load the EXE knowing nothing said DLL(s). It is then application defined as to how to handle success or failure of LoadLibrary() and /or GetProcAddress().
It is worth noting at this point, that libraries like the C-Runtime may be provided in DLL form (dynamic library) or static form (static library). If you link to one of these libraries statically, there will be no DLL import section in the built EXE's PE header and no function stubs to resolve at runtime for that library. Instead of stubs, these symbols (functions and/or data variables) become part of the EXE. Static library functions and/or data are copied into the EXE and are assigned relative addresses explicitly by the linker; no different than if those symbols were implemented directly by the EXE. Additionally, there will be no LoadLibrary() or GetProcAddress() resolution either implicitly (by the Windows loader) or explicitly in code for these functions as they will be directly present and self-contained within the final EXE. As a side-note, debugging symbols may be used in this case to try and differentiate between EXE implemented functions and library implemented functions (should you care) but this is highly dependent on the settings used to build both the EXE and the static library.
With terminology cleared up, let me attempt to answer your question! :)
Let me also add I'm not going to go into the specifics of bound and unbound import symbols for a module's import section because this distinction has nothing to do with the original question and have more to do with speeding up the work done by the Windows loader. If you are interested in those details however, you can read up on Microsoft's PE COFF Specification.
To see if an EXE is statically bound to a DLL, you can either parse the PE headers yourself to locate the DLL imports section or use one of dozens of tools to do this for you, such as Dependency Walker. If you load your EXE in Dependency Walker for example, you will see a list of all statically bound DLLs in the top-left pane underneath the EXE itself. If any of these DLLs are not found at runtime, the program will fail to load. In the right pane, top table, you will see symbols (e.g. functions) that are referenced in the EXE for the selected DLL. All of these symbols must additionally be found for the EXE to load. The lower table simply shows all of the symbols exported by the DLL, referenced or not.
If the EXE uses dynamic binding (also called it manual binding) for a given DLL, there will be no import section for that DLL and thus you won't see it referenced in tools like Dependency Walker. BUT, you can click on KERNEL32.DLL in Dependency Walker (all EXEs will have this dependency, though there are exceptions to this rule I won't get in to here) and locate references to LoadLibrary() and GetProcAddress(). Unfortunately most EXEs reference these functions in boilerplate code such as the C-Runtime so this won't tell you too much.
If you want to dig deeper into trying to figure out which DLLs are manually loaded by an application, the first thing to try is to and locate that DLL name string by searching the EXE for the DLL name. Note that the DLL name string need not end in ".DLL" as LoadLibrary() automatically assumes this extension if not provided. The standard tool for searching for strings within a binary module is Sysinternals Strings. This works great for modules that make no attempt to hide what they are doing.
With that said, obfuscated code (found in unpackers, viruses and the like) may obfuscate or encrypt DLL names as well as the functions referenced. Code may even choose to resolve LoadLibrary() and GetProcAddress() at runtime to further hinder efforts to figure out what they are doing. Your best bet in these situations is to use a tool like Sysinternals Process Monitor or a debugger with verbose logging enabled to watch the DLLs being loaded as the program runs. You can also use a disassembler (such as IDA) to try and piece together what the code is doing. To find out what DLL symbols are being used, you might start the EXE in a debugger and wait for the initial break at the entry-point. Then add a breakpoint on the first instruction in KERNEL32.GetProcAddress. Run the program. When that breakpoint is hit, the stack arguments will contain the symbol trying to be resolved.
As you can see, if an application resolves DLL symbols manually (dynamic binding), the process of figuring out what DLLs are being referenced is not as straightforward.
I have learned that you can:
Convert a .DLL file into a .DEF file, which includes its exports
(Edit: This doesn't work with many conventions)
Convert a .DEF file into a .LIB file, which you can use to link to the DLL
Why can't (most) linkers link to a DLL given only a .DEF file, instead of a .LIB file?
Ultimately, the answer here is 'because noone wanted it badly enough and it doesn't really help anything'.
The DEF file is an input file that creates an import lib for the DLL. And then, later, when the DLL is being consumed by another link, the importlib is itself an input. The importlib looks like something special on the outside, but when you look at the inside it's really just a slightly special lib with objects in it.
It totally would be possible to modify the linker to take a def file (or a DLL, for that matter) directly.
But the design centre of the linker is that it takes objects as inputs and outputs a PE executable. So taking a DEF or DLL as an input goes outside the design pattern.
Beyond that it'd be rather pointless - allowing the linker to take a DEF file or DLL as input would neither enable any important new scenarios, nor does leaving this feature out block anything. Converting a DEF file you have (even without the actual DLL) into a usable importlib is the work of a few moments (simply create fake empty function for each DEF entry and link that). So there's no reason to add the ability to link a DEF file directly.
Martyn
In terms of MSVC, .lib files are always static libraries. They get linked in as a compilation unit along with all your compiled .c/.cpp files, so all of the library's code is included in your final executable.
Some .lib files, however, (in particular most of the Windows system ones) merely contain stubs which tell the OS to load the desired DLL at loadtime and then the stubs route function calls to the DLL. But, those stubs are statically linked into your executable. Your program will then use DLLs (and gain all the advantages and disadvantages thereof), but since the named DLL functions it requires are happily located in the .lib (and thus actually located in the executable itself), your code doesn't have to know it's using a DLL (specifically using declspec(dllimport)).
A .def file is merely used as a sort of "settings" or "configuration" file during the creation of a .dll to specify what functions the file should export. It cannot be linked to, as it doesn't really describe anything that the linker understands.
You do not convert a dll to a DEF file. The DEF just indicates which dll functions will be accessible from the outside, exported.
From the docs:
A DLL file has a layout very similar to an .exe file, with one
important difference — a DLL file contains an exports table. The
exports table contains the name of every function that the DLL exports
to other executables. These functions are the entry points into the
DLL; only the functions in the exports table can be accessed by other
executables. Any other functions in the DLL are private to the DLL.
The exports table of a DLL can be viewed by using the DUMPBIN tool
with the /EXPORTS option.
You can export functions from a DLL using two methods:
Create a module definition (.def) file and use the .def file when
building the DLL. Use this approach if you want to export functions
from your DLL by ordinal rather than by name.
Use the keyword __declspec(dllexport) in the function's definition.
When exporting functions with either method, make sure to use the
__stdcall calling convention.
Use the provided link to learn more about exporting from your dll's.
I think you got down voted because your point is not really clear, at least not to me. Also check this. It explains how to choose the export method.
Mehrdad, this isn't always a question of how to LINK to a DLL, as I, personally have NEVER linked a DLL using a .DEF file. What I HAVE done is take someone else's DLL, and very painstakingly constructed a header file, or rather, function prototypes that I could use with
LoadLibrary() in C, Declare Function ... Lib "Foo.dll" Alias "OrdinalName" in VB, and
[DllImport()] in C#.
Of course, this is RARELY done, as if you are using a DLL for something, normally you have permission to do so, and the authors provide the .lib's, and the headers to go with the binary DLL file.
I've never done the exact techniques you speak of, by converting a .DEF info a .LIB, etc... But, I suppose it would be easy to take a lib, or the DLL itself and export .DEF from it. Now, THAT I actually HAVE done, in a project where the DLL code was built with a vbScript that took code from the main project, and created an API out of all the existing, compiled, and tested code. This level of complication was only done because I had no idea what functions were going to BE in the DLL, as the main project could change at any time, so a static .DEF file would have never worked. So, I had to build the DLL once, capture the dimpbin /exports, undecorate the functions, and then build the .DEF file, and re-link the DLL.
If you find yourself in that type of situation, perhaps you need to re-think your original designs, and fix the problem from there...
As for .LIB files, USUALLY you'd only NEED those for static linkage, but they are also used when the .H file is available, often making debugging a just a little nicer...
We create a DLL for other applications to load and use some of the functionality in the application. The DLL has dependency on the the actual path where it is loaded from.
<product_home>/bin/<DLL is here>
|
|----/configdir/configfile
|----/lib/<java jarfiles>
It needs the product_home location to read config files and load jar files etc
My windows application proloads a special DLL. I need to find actual path to a loaded DLL with in the process and use it to set a "HOME" variable. This will be used in rest of the processing. Using an externally set environment variable some time fails when there are multiple versions of dll present on the machine. To me it looks like DLL can figure out its own "product_home" as long it can get the actual loaded location.
The DLL This article Get Your DLL's Path/Name provides one such way- (yet to try it successfully. The generated exe crashes). Is this the correct approach?
Either I don't understand your need, or the link you mention is not what you need. If I understand you correctly, you'd like to get the full path of a certain DLL loaded by the process. So, say that DLL is "kernel32.dll", you'd like to get "c:\windows\system32\kernel32.dll". Please correct me if I'm wrong.
If that's what you want, the easiest way to do that would be:
HMODULE hModule = GetModuleHandle(_T("kernel32.dll"));
TCHAR dllPath[_MAX_PATH];
GetModuleFileName(hModule, dllPath, _MAX_PATH);
Failures checks omitted for brevity - read more about GetModuleHandle and GetModuleFileName.
I want to know the exact difference between the dll and exe file.
I don't know why everybody is answering this question in context of .NET. The question was a general one and didn't mention .NET anywhere.
Well, the major differences are:
EXE
An exe always runs in its own address space i.e., It is a separate process.
The purpose of an EXE is to launch a separate application of its own.
DLL
A dll always needs a host exe to run. i.e., it can never run in its own address space.
The purpose of a DLL is to have a collection of methods/classes which can be re-used from some other application.
DLL is Microsoft's implementation of a shared library.
The file format of DLL and exe is essentially the same. Windows recognizes the difference between DLL and EXE through PE Header in the file. For details of PE Header, You can have a look at this Article on MSDN
EXE:
It's a executable file
When loading an executable, no export is called, but only the module entry point.
When a system launches new executable, a new process is created
The entry thread is called in context of main thread of that process.
DLL:
It's a Dynamic Link Library
There are multiple exported symbols.
The system loads a DLL into the context of an existing process.
For More Details: http://www.c-sharpcorner.com/Interviews/Answer/Answers.aspxQuestionId=1431&MajorCategoryId=1&MinorCategoryId=1
http://wiki.answers.com/Q/What_is_the_difference_between_an_EXE_and_a_DLL
Reference: http://www.dotnetspider.com/forum/34260-What-difference-between-dll-exe.aspx
The difference is that an EXE has an entry point, a "main" method that will run on execution.
The code within a DLL needs to be called from another application.
There are a few more differences regarding the structure you could mention.
Both DLL and EXE share the same file structure - Portable Executable, or PE. To differentiate between the two, one can look in the Characteristics member of IMAGE_FILE_HEADER inside IMAGE_NT_HEADERS. For a DLL, it has the IMAGE_FILE_DLL (0x2000) flag turned on. For a EXE it's the IMAGE_FILE_EXECUTABLE_IMAGE (0x2) flag.
PE files consist of some headers and a number of sections. There's usually a section for code, a section for data, a section listing imported functions and a section for resources. Some sections may contain more than one thing. The header also describes a list of data directories that are located in the sections. Those data directories are what enables Windows to find what it needs in the PE. But one type of data directory that an EXE will never have (unless you're building a frankenstein EXE) is the export directory. This is where DLL files have a list of functions they export and can be used by other EXE or DLL files. On the other side, each DLL and EXE has an import directory where it lists the functions and DLL files it requires to run.
Also in the PE headers (IMAGE_OPTIONAL_HEADER) is the ImageBase member. It specifies the virtual address at which the PE assumes it will be loaded. If it is loaded at another address, some pointers could point to the wrong memory. As EXE files are amongst the first to be loaded into their new address space, the Windows loader can assure a constant load address and that's usually 0x00400000. That luxury doesn't exist for a DLL. Two DLL files loaded into the same process can request the same address. This is why a DLL has another data directory called Base Relocation Directory that usually resides in its own section - .reloc. This directory contains a list of places in the DLL that need to be rebased/patched so they'll point to the right memory. Most EXE files don't have this directory, but some old compilers do generate them.
You can read more on this topic # MSDN.
This answer was a little more detailed than I thought but read it through.
DLL:
In most cases, a DLL file is a library. There are a couple of types of libraries, dynamic and static - read about the difference. DLL stands for dynamic link library which tells us that it's a part of the program but not the whole thing. It's made of reusable software components (library) which you could use for more than a single program. Bear in mind that it's always possible to use the library source code in many applications using copy-paste, but the idea of a DLL/Static Library is that you could update the code of a library and at the same time update all the applications using it - without compiling.
For example:
Imagine you're creating a Windows GUI component like a Button. In most cases you'd want to re-use the code you've written because it's a complex but a common component - You want many applications to use it but you don't want to give them the source code You can't copy-paste the code for the button in every program, so you decide you want to create a DL-Library (DLL).
This "button" library is required by EXEcutables to run, and without it they will not run because they don't know how to create the button, only how to talk to it.
Likewise, a DLL cannot be executed - run, because it's only a part of the program but doesn't have the information required to create a "process".
EXE:
An executable is the program. It knows how to create a process and how to talk to the DLL. It needs the DLL to create a button, and without it the application doesn't run - ERROR.
hope this helps....
Both DLL and EXE are Portable Executable(PE) Formats
A Dynamic-link library (DLL) is a library and therefore can not be executed directly. If you try to run it you will get an error about a missing entry point. It needs an entry point (main function) to get executed, that entry point can be any application or exe. DLL binding occurs at run-time. That is why its called "Dynamic Link" library.
An Executable (EXE) is a program that can be executed. It has its own entry point. A flag inside the PE header indicates which type of file it is (irrelevant of file extension). The PE header has a field where the entry point for the program resides. In DLLs it isn't used (or at least not as an entry point).
There are many software available to check header information. The only difference causing both to work differently is the bit in header as shown in below diagram.
EXE file has only single main entry means it is isolated application, when a system launches exe, a new process is created while DLLs have many entry points so when application use it no new process started, DLL can be reused and versioned. DLL reduces storage space as different programs can use the same dll.
Dll v/s Exe
1)DLL file is a dynamic link library which can be used in exe files and
other dll files.
EXE file is a executable file which runs in a separate
process which is managed by OS.
2)DLLs are not directly executable . They are separate files containing functions that can be called by programs and other DLLs to perform computations and functions.
An EXE is a program that can be executed . Ex :Windows program
3)Reusability
DLL: They can be reused for some other application. As long as the coder knows the names and parameters of the functions and procedures in the DLL file .
EXE: Only for specific purpose .
4)A DLL would share the same process and memory space of the calling application while an
EXE creates its separate process and memory space.
5)Uses
DLL: You want many applications to use it but you don't want to give them the source code You can't copy-paste the code for the button in every program, so you decide you want to create a DL-Library (DLL).
EXE: When we work with project templates like Windows Forms Applications, Console Applications, WPF Applications and Windows Services they generate an exe assembly when compiled.
6)Similarities :
Both DLL and EXE are binary files have a complex nested structure defined by the Portable Executable format, and they are not intended to be editable by users.
Two things: the extension and the header flag stored in the file.
Both files are PE files. Both contain the exact same layout. A DLL is a library and therefore can not be executed. If you try to run it you'll get an error about a missing entry point. An EXE is a program that can be executed. It has an entry point. A flag inside the PE header indicates which file type it is (irrelevant of file extension). The PE header has a field where the entry point for the program resides. In DLLs it isn't used (or at least not as an entry point).
One minor difference is that in most cases DLLs have an export section where symbols are exported. EXEs should never have an export section since they aren't libraries but nothing prevents that from happening. The Win32 loader doesn't care either way.
Other than that they are identical. So, in summary, EXEs are executable programs while DLLs are libraries loaded into a process and contain some sort of useful functionality like security, database access or something.
The .exe is the program. The .dll is a library that a .exe (or another .dll) may call into.
What sakthivignesh says can be true in that one .exe can use another as if it were a library, and this is done (for example) with some COM components. In this case, the "slave" .exe is a separate program (strictly speaking, a separate process - perhaps running on a separate machine), but one that accepts and handles requests from other programs/components/whatever.
However, if you just pick a random .exe and .dll from a folder in your Program Files, odds are that COM isn't relevant - they are just a program and its dynamically-linked libraries.
Using Win32 APIs, a program can load and use a DLL using the LoadLibrary and GetProcAddress API functions, IIRC. There were similar functions in Win16.
COM is in many ways an evolution of the DLL idea, originally concieved as the basis for OLE2, whereas .NET is the descendant of COM. DLLs have been around since Windows 1, IIRC. They were originally a way of sharing binary code (particularly system APIs) between multiple running programs in order to minimise memory use.
An EXE is visible to the system as a regular Win32 executable. Its entry
point refers to a small loader which initializes the .NET runtime and tells
it to load and execute the assembly contained in the EXE.
A DLL is visible to the system as a Win32 DLL but most likely without any
entry points. The .NET runtime stores information about the contained
assembly in its own header.
dll is a collection of reusable
functions where as an .exe is an
executable which may call these
functions
An exe is an executible program whereas A DLL is a file that can be loaded and executed by programs dynamically.
● .exe and dll are the compiled version of c# code which are also called as
assemblies.
● .exe is a stand alone executable file, which means it can executed directly.
● .dll is a reusable component which cannot be executed directly and it requires
other programs to execute it.
For those looking a concise answer,
If an assembly is compiled as a class library and provides types for other assemblies to use, then it has the ifle extension .dll (dynamic link library), and it cannot be executed standalone.
Likewise, if an assembly is compiled as an application, then it has the file extension .exe (executable) and can be executed standalone. Before .NET Core 3.0, console apps were compiled to .dll fles and had to be executed by the dotnet run command or a host executable. - Source
Difference in DLL and EXE:
1) DLL is an In-Process Component which means running in the same memory space as the client process. EXE is an Out-Process Component which means it runs in its own separate memory space.
2) The DLL contains functions and procedures that other programs can use (promotes reuability) while EXE cannot be shared with other programs.
3) DLL cannot be directly executed as they're designed to be loaded and run by other programs. EXE is a program that is executed directly.
The major exact difference between DLL and EXE that DLL hasn't got an entry point and EXE does. If you are familiar with c++ you can see that build EXE has main() entry function and DLL doesn't :)
My question is not exactly the same as this one (it's not theoretical, there is only a main thread without message loop, InitInstance and ExitInstance are no fitting calls).
I'm using a console app without message loop; this app loads an exe with the LoadLibrary function, so that it can use its exported functions. Bad news: the DllMain function of the exe is not called (and I verified the symbols tables, using a def file, DllMain appears correctly); the doc says it's called if the loaded module is a DLL (too bad).
What are the conditions (if they exist) which could lead to the execution of the exe's DllMain function when LoadLibrary is called (and maybe again when FreeLibrary is called)?
Best regards
The most obvious condition is that the process calling LoadLibrary() explicitly gets GetProcAddress("DllMain") and then calls it.
The conditions are:
1) Binary being loaded was compiled as DLL (when using gcc/ld it means using --shared option; If you use --shared, resulting file will be a dll, and will not run, see below)
2) IMAGE_FILE_DLL is set in PE file header of the binary file being loaded. If it is set, file is a dll, and Windows linker will call its DllMain() function for you when it links this file to your program (doesn't matter how it's linked - LoadLibrary() at runtime or -llibraryname at compile time). For that the file must also satisfy (1). But with this flag the binary file being loaded will not be runnable. If IMAGE_FILE_DLL is not set, DllMain() will not be called when file is loaded into your program.
Compiling dll with --shared and then manually removing IMAGE_FILE_DLL from its header (i.e. by using hex editor) will not work - when you run it, only DllMain() will be executed, and fdwReason will be a non-defined number (0x28ffd4 on my machine).
Update
All DLL and EXE files on Windows are PE files, the difference is how they are linked, and which flags are set in their headers. That is why i write file being loaded, not dll being loaded.
The last paragraph also describes the scenario where you compile the file as dll, and then turn it into exe by messing with its header. It doesn't work.
Naming has nothing to do with it (you can choose any name, and with some pexports+dlltool tinkering you can create an import library for an .exe file and be able to link it as -lexenamewithoutextension
To clarify:
if you compile it without --shared:
IMAGE_FILE_DLL will not be set in it, it will be runnable, but DllMain() will NOT be called when you link it.
if you compile it with --shared:
IMAGE_FILE_DLL will be set in it, it will NOT be runnable, but DllMain() will be called when you link it.
if you compile it without --shared, then switch on the IMAGE_FILE_DLL flag in it manually:
it will NOT be runnable anymore, and no DllMain() will be called when you link it.
if you compile it with --shared, then switch off the IMAGE_FILE_DLL flag in it manually:
it will be runnable, but DllMain() will be executed instead of main(), and DllMain() will NOT be called when you link it.
Indeed the Name "DllMain" of the function is ignored by Windows completely (the old Windows API documentation of Windows NT 3.x stated this explicitly).
When a DLL is loaded not the function DllMain() but the function that is located at the file's entry point is called.
Of course the linker will create the DLL file in a way that DllMain() is this function.
However for EXE files the entry function (that will call WinMain()) is located at the entry point.
So it is obvious that Windows cannot call this function when loading the EXE file as DLL.
Completing the good answer of MSalters:
So, then, call the "fake" DllMain with DLL_XXX_ATTACH just after LoadLibrary and with DLL_XXX_DETACH just before FreeLibrary, and make manually the other calls.
Another following implementation would be to build and load an interface DLL which could callback automatically the EXE on its fake DllMain (I don't know if it could work); but it may be more complicated than just manually calling the fake DllMain in a number of cases. (can't LoadLibrary in a DllMain)